This invention relates to electronic assembly technology and more specifically to packaging micro-electromechanical systems (MEMS) or micro-optoelectromechanical devices (MOEMS).
New photonic devices are in development that use micromechanical elements. In principal, micromechanical elements can be built on a variety of platforms. However, the substrate of choice is typically a semiconductor wafer, e.g. silicon. Highly and often elegantly engineered silicon processing can be used to make new device structures that combine the mechanical and optical properties of silicon. An advanced technology, silicon optical bench technology, has been developed to implement this approach. Typically the micromechanical devices or subassemblies are formed in large integrated arrays, referred to here as MEMS, to perform a common function in a parallel mode. The substrate for the arrays is usually a silicon wafer or a large silicon chip. In most instances the MEMS device arrays comprise photonic devices, and are accessed with optical I/O signals.
Among the most promising of the photonic MEMS devices are optical cross connect devices. These may be used in optical networking for routing optical signals from one array of optical channels to another. Optical cross connects are typically made in the form of compact arrays of micromechanical mirrors. In an input array with a two dimensional, or 2-D, architecture usually a linear array of optical waveguides are arranged to address the mirror array, which steers optical beams from the input array to a corresponding output array of optical waveguides. The input and output optical channels may be optical waveguides in an optical integrated circuit, or may be arrays of optical fibers.
These optical cross connect devices can switch one of a large number of optical inputs between a selected one of a large number of optical outputs. For example, a 10 fiber input array used with a 10 fiber output array has the capacity to make 100 individual connections. Each channel typically has tens or, in future systems, hundreds of channels of wavelength division multiplexed (WDM) signals. The information handling capacity of such a switch is extremely large.
State of the art optical networking systems require large compact arrays of micromechanical mirrors. The micromechanical mirrors are electrically addressed, and mirror tilt is controlled by selectively applied electrostatic fields. In a standard optical networking system, for n input fibers in a 2-D implementation an n2 mirror array is used. Each input fiber accesses an associated row of, for example, ten mirrors and each of the ten mirrors addresses one of ten output fibers. In a typical operating cross connect, for example, the first three mirrors are not activated, i.e. do not intersect the beam path, and the fourth is electrically tilted to intersect the beam path and steer the beam to its associated fiber. In this way the first fiber can address a selected one of ten mirrors and thus a selected one of ten fibers. This n2 mirror array requires two tilt positions, on and off. A more efficient mirror arrangement uses 2n mirrors for the same 10xc3x9710 switch. It operates by steering the optical beam to one of ten positions, and has two way tilt capability.
Mirrors for optical cross connects in current state of the art devices may be formed using the silicon bench technology mentioned above. A silicon wafer platform is used as the support substrate, and the mirrors are fabricated on the silicon platform using known micromechanical device fabrication techniques. Some of these techniques have been developed for optical modulator devices such as the Mechanical Anti-Reflection Switch (MARS) device. See e.g. U.S. Pat. No. 5,943,155 issued Aug. 24, 1999, and U.S. Pat. No. 5,943,571, issued Sep. 7, 1999. The fabrication approach used in this technology is to fabricate a layered structure on a silicon substrate, with the top layer of a reflecting material, and dissolve away the intermediate layer(s) leaving a suspended reflector.
It should be evident from the foregoing description that optical alignment in MEMS assemblies, i.e. packaged MEMS device arrays, is extremely critical. Alignment defects occur during manufacture and also arise in the use environment, i.e. after manufacture. The former can be dealt with by process control and thorough testing. The latter however, are more abstruse and unpredictable. They may occur as the result of mechanical perturbations in the assembly caused, for example, by handling. More typically, they result from differential thermal expansion due to temperature variations in the use environment. This produces strains which may impair the precise alignment of optical elements in the assembly. To preserve critical alignment, the MEMS device arrays may be mounted in a robust container with a suitable transparent opening for accessing the array with light signals. In many cases it is desirable to mount them in hermetic packages. However these protective packages often exacerbate the thermo-mechanical problems caused by differential thermal expansion.
Use of standard epoxy/glass printed circuit boards for packaging MEMS devices has proven less than satisfactory. Large MEMS arrays require complex and efficient interconnection arrangements as well as demanding thermomechanical performance. As indicated above, thermomechanical performance is required in general for hermetic packages, and is particularly important for photonic devices that require precise optical alignment. Also, hermeticity can only be achieved using materials with sufficiently low diffusion rates for air and water vapor. Such materials are generally restricted to metals, glasses and ceramics. While epoxy printed circuit boards offer good interconnect capability, and at reasonable cost, the thermomechanical and hermetic properties of epoxy boards are less than desired for this application.
Ceramic chip carrier technology is typically the packaging technology of choice for high quality state of the art IC devices. It is especially suitable for hermetic packages, due to high thermomechanical performance and low diffusion rates. However, ceramic packaging is costly, especially where high density interconnection and routing is required.
We have developed a packaging technology for MEMS assemblies in which the MEMS device arrays are mounted on a ceramic platform and are then packaged in a hybrid package. The hybrid package may be hermetically sealed. The hybrid package uses a ceramic insert as the primary MEMS device enclosure. The ceramic insert is mounted on a polymer printed wiring board, which provides both support and electrical interconnection for the ceramic insert. Optical access to the MEMS device is through a transparent window hermetically sealed to the ceramic insert. The use of a ceramic primary enclosure for the MEMS device array substantially eliminates thermomechanical instabilities, and provides thermomechanical and hermetic performance for the elements that require it. The interconnection function of the package is implemented using standard epoxy printed circuit technology. This yields high interconnection performance at low cost.